Bacteroides is a genus of Gram-negative, bacillus bacteria. Bacteroides are non-endospore forming, anaerobes that comprise a significant portion of the normal human flora, particularly in the intestinal tract. Here they play an essential part in processing complex molecules into simpler ones in the host's intestines.

Bacteroides are significant as an alternative fecal indicator organism since they make up such a large amount of the normal fecal bacterial flora, have a high degree of host specificity and typically have a low potential to grow in the general environment. Due to their sheer numbers, the chance of finding Bacteroides over other traditional fecal indicators, such as E. coli, are greatly increased.